Flip chip microelectronic assembly is the direct electrical connection of face-down (hence, “flipped”) electronic components onto substrates, circuit boards, or carriers, using conductive bumps formed on chip or die bond pads. In contrast, wire bonding, a conventional technology, uses face-up chips with a wire connection to each chip bond pad. Flip chip packaging has several advantages over conventional packages including size, performance, flexibility, reliability, and cost.
Typically, flip chip components are semiconductor devices, but manufacturers use the technology as well with components such as passive filters, detector arrays, and sensor devices. Flip chip is also referred to Direct Chip Attach (DCA), a more descriptive term, because the chip is directly attached to the substrate, board, or carrier by the conductive bumps.
In flip chip devices, the conductive bumps serve several functions. Electrically, the bumps provide the conductive path from the chip to the substrate. Thermally, the bumps provide a heat conductive path to carry heat away from the chip to the substrate. Mechanically, the bumps become part of the physical mount between the chip and the substrate. Further, the bumps provide a spacer or stand-off feature that prevents electrical contact between the chip and substrate conductors.
There are three general stages in manufacturing flip chip devices. These include bumping the chip or wafer, attaching the bumped chip to the board or substrate, and in most cases, filling the remaining space under the chip with an electrically non-conductive material. The present invention pertains to the first general stage of bumping.
Prior art methods of forming flip chip bumps include solder bumping, plated bumping, stud bumping, and adhesive bumping. A solder bumping process first requires that an under bump metallization (UBM) be placed onto the bond pads by sputtering, plating, or other means. Conductive solder bumps are then deposited over the UBM by evaporation, electroplating, screen printing solder paste, or needle-depositing.
In plated bump technology, wet chemical processes are used to remove native oxide films and to plate conductive metal bumps onto the bond pads. For example, plated nickel-gold bumps are formed by electroless nickel plating of bond pads, which comprise, for example, aluminum. After plating a desired thickness of nickel, an immersion gold layer is added for protection.
In the stud bump process, a modified standard wire bonding technique is used with gold wire being typical. This technique makes a gold ball for wire bonding by melting the end of a gold wire to form a sphere. The gold ball is attached to the chip bond pad as the first part of a wire bond. To form gold bumps instead of wire bonds, wire bonders are modified to break off the wire after attaching the ball to the chip bond pad.
In the adhesive bump flip chip process, stencils are used to form conductive adhesive bumps on UBM layers overlying chip bond pads. The cured adhesive acts as conductive bumps. An additional layer of conductive adhesive is used to attach the chip to a next level of assembly.
As is apparent from the above description, the various prior art methods for forming flip chip conductive bumps involve extensive processing steps and added materials that are expensive. Also, these processes involve additional or complex wafer handling steps that can cause wafer breakage and/or damage, which in turn, detrimentally impacts yield and device performance. Moreover, many of the fabrication steps used in the prior art processes use environmentally restrictive and harmful chemicals.
Accordingly, a need exists for a cost effective and reliable bump structure and method of manufacture that overcome the issues of the prior art including those noted above.